The present invention relates to a method and apparatus for mapping all colors of an input image to a color gamut that can be reproduced by an output device in the field of color matching between different color reproduction devices. More particularly, the present invention relates to a method and apparatus for color gamut mapping in which an image can be reproduced with an optimum appearance by evaluating color matching using new color differences defined taking spatial frequency characteristics of the visual sense of a person.
The entire contents of Japanese Patent Application No. 9-53566 filed on Mar. 7, 1997 are incorporated herein by reference.
Various types of apparatuses have been developed as media for color reproduction including color displays, dye-sublimation printers, ink-jet printers, photographs, and commercial printers. In general, color gamuts which can be reproduced by such apparatuses significantly vary depending on the methods of color reproduction (color mixture), the methods for tone reproduction (halftone dot method, dithering, error diffusion method, etc), and differences in the spectral characteristics of the primary colors used. For example, it is known that the color gamut of a display is considerably wider than that of a dye-sublimation type printer. When color matching is performed between two apparatus having color gamuts of different sizes as described above, it is inevitable that some colors are unreproducible for one of the devices. Therefore, a problem arises in that it is necessary to find how to allow reproduce such colors out of such a color gamut at a device of interest.
In general, the visual system of a person who observes an image is characterized in that it has a difficulty in detecting a difference of a color from an original color for each picture element because it maintains the relationship between adjoining pixels (spatial frequency characteristics of an image) when such adjoining pixels have some complicated context an the case of a natural image. On the other hand, it detects a difference in one kind of color such as on a color chip more easily than it does on a natural image because the image of such a single color has constant frequency characteristics.
However, methods of mapping a color gamut proposed in the past pays no attention on such spatial frequency characteristics of the human visual system at all. Further, since those methods are used for color gamut mapping between an original image and a reproduced image on a pixel-by-pixel basis, a problem has been pointed out in that there is no way to provide a practical method of performing color gamut mapping on a natural image.
As described, it has been difficult to provide a method of performing color gamut mapping on a natural image properly, and there is a need for a more practical technique for performing color gamut mapping on an image that takes the spatial frequency characteristics of the human visual system into consideration.